Dopamine agonists stimulate protein carboxymethylation in striatal synaptosomes

Protein carboxyl methylase (PCM; E.C. 2.1.1.24) catalyzes the formation of labile carboxyl methyl esters using S-adenosylmethionine as a methyl donor. PCM activity is high in the central nervous system and is present in synaptosomes, suggesting a role for this enzyme in presynaptic events. Incubation of striatal synaptosomes with apomorphine (10^-7 to 10^-5 M) significantly increased methylester formation in a dose-related manner. This stimulatory effect of apomorphine was blocked by preincubation with the dopamine antagonist (+)-butaclamol (10^-5 M), but not by (-)-butaclamol (10^-5 M), an inactive isomer, suggesting that the effects of apomorphine were mediated by specific dopamine receptors. Dopamine (10^-5 M) and the selective dopamine autoreceptor agonist, 3(3-hydroxyphenyl)-N-n-propylpiperidine (10^-5 M), also significantly increased methyl ester formation in striatal synaptosomes. Synaptosomal uptake of [³H]-S-adenosylmethionine was not affected by apomorphine (10^-5 M). Additional experiments indicated that these drug effects were mediated by an interaction with dopamine autoreceptors. When synaptosomes were prepared from striata of rats pretreated i.c.v. with 6-hydroxydopamine (200 μg), the stimulatory effects of apomorphine were abolished. Synaptosomes prepared from the prefrontal cortex, a region devoid of dopamine autoreceptors, and the hippocampus did not exhibit enhanced methylester formation when exposed to apomorphine. When purified PCM was assayed in the presence of 10^-5 M apomorphine, there was no alteration in either K(m) or V(max) for [³H]-S-adenosylmethionine, indicating that apomorphine was not directly activating PCM. Similarly, 10^-5 M apomorphine did not affect methylation of calmodulin by purified PCM. Acidic gel electrophoresis was used to identify methyl acceptor proteins in striatal synaptosomes and indicated several bands of acceptor proteins, including a prominent peak around 16,000 daltons, suggesting that calmodulin is a substrate for PCM in synaptosomes.